In the electrostatographic process of producing full color images on an image receiver sheet (e.g., a sheet of plain paper), a plurality of toner images of different color (e.g., cyan, magenta, yellow and black) are produced on a reusable dielectric (usually photoconductive) recording element. Such images are then transferred to the receiver sheet, one atop the other and in registration, to produce the desired full color image. As may be readily appreciated, image quality is determined, to a large extent, by the degree to which the transferred images are in registration on the receiver sheet. In electrophotographic systems in which the transferable images are formed on a continuously moving recording element, precise registration requires that each transferable image arrive at an image transfer zone at the precise instant that the desired portion of the receiver sheet to which the image is to be transferred arrives. Precise registration is no trivial problem, especially when attempting to produce full color images of photographic quality from a series of transferable toner images.
In the commonly assigned U.S. application Ser. No. 488,546, now U.S. Pat. No. 5,040,026 filed on Mar. 5, 1990 in the names of F. S. Jamzadeh et al., entitled MULTICOLOR IMAGE FORMING APPARATUS, there is disclosed an electrophotographic apparatus in which transferable toner images of different colors are formed on the outer surface of a photoconductive drum. These images are transferred, one at a time, to a receiver sheet to provide a full color image thereon. Such apparatus comprises a rotatably mounted transfer drum which is adapted to support and transport the receiver sheet on a portion of its outer surface. The transfer drum is spring biased toward the photoconductive drum so that the respective outer surfaces of the drums are in physical contact. The photoconductive drum is rotatably driven and, when a receiver sheet is positioned in the nip between the drums to effect image transfer, the photoconductive drum frictionally drives the transfer drum through the receiver sheet. According to a preferred embodiment, a caming mechanism serves to separate the drums between successive image transfers and, during this interframe period, and independent stepper motor serves to rotatably drive the transfer drum to re-index the transfer drum with the photoconductive drum. An encoding system is provided to monitor the relative positions of the drums so that the stepper motor can, at the appropriate time, accelerate the transfer drum to the same speed as the photoconductive drum and present the receiver sheet to the transfer nip in timed relation with the arrival of the transferable image on the photoconductive drum.
In comparison to systems employing two independent stepper motors to continuously rotate the process and transfer drums during image transfer, the above technique of using the photoconductive drum to drive the transfer drum during image transfer and of re-indexing the transfer drum after each image transfer is highly advantageous in the degree of registration it provides. Such image transfer apparatus is capable of achieving image registration accuracy of better than 0.0025 cm. over the entire image area. As noted above, however, the preferred apparatus disclosed in this application uses two separate precision drive motors, one to continuously drive the photoconductive drum, and the other to accelerate and control the velocity of the transfer drum to achieve re-indexing during the interframe period.
In the commonly assigned U.S. application Ser. No. 532,831 now U.S. Pat. No. 5,021,835 filed on Jun. 4, 1990 in the name of K. M. Johnson, entitled MULTICOLOR IMAGING APPARATUS WITH IMPROVED TRANSFER MEANS, there is disclosed an electrophotographic recording apparatus having a transfer mechanism somewhat similar to that described above. In this apparatus, however, the two drums (i.e., the photoconductive image process drum and transfer drum) are physically separated by a distance less than the compacted thickness of the receiver sheet. As in the above-described system, the photoconductive drum drives the transfer drum through the receiver sheet during image transfer. Also, the preferred apparatus disclosed uses two precision drive motors, one for each drum.
In the commonly assigned U.S. application Ser. No. 685,251, filed on Apr. 15, 1991 in the name of Kevin M. Johnson and entitled OVERRIDABLE WORM GEAR DRIVE FOR MULTICOLOR IMAGE FORMING APPARATUS, there is disclosed an electrophotographic apparatus of the above type in which the transfer and photoconductive drums are slightly spaced apart. The photoconductive drum is continuously driven by a precision stepper motor, and the transfer drum is independently driven by a worm gear drive. During transfer, when the transfer drum is rotatably driven by the photoconductive drum through the receiver sheet, the worm gear drive is overriden, assuring that it does not interfere with the advancement of the transfer drum by the other drum. Here, again, re-indexing of the transfer drum is achieved during the interframe period by using the worm gear drive motor to accelerate the transfer drum at the proper time and to the appropriate speed to achieve image registration at the transfer zone (i.e., the nip between the drums). As in the above systems, two precision motors are used, one for each drum.